A reciprocating-type variable capacity compressor used in an automotive air conditioning system, for example, includes a housing having a discharge chamber, a suction chamber, a crank chamber and cylinder bores defined therein. A drive shaft extending through the crank chamber is coupled with a swash plate such that the swash plate is tiltable relative to the drive shaft. A conversion mechanism including the swash plate converts rotation of the drive shaft to reciprocating motion of pistons received in the respective cylinder bores. The reciprocating motion of each piston causes a series of processes to take place, the processes including a suction process in which a working fluid is sucked from the suction chamber into the corresponding cylinder bore, a compression process in which the sucked working fluid is compressed, and a discharge process in which the compressed working fluid is discharged to the discharge chamber.
The stroke length of the individual pistons, that is, the discharge capacity of the compressor, can be varied by changing the pressure (control pressure) in the crank chamber. In order to control the discharge capacity, a capacity control valve is inserted in an admission passage communicating the discharge chamber with the crank chamber, and a constriction is formed in a bleeding passage communicating the crank chamber with the suction chamber.
As methods for controlling the discharge capacity, a suction pressure control scheme has been known. In some systems configured to execute the suction pressure control, the capacity control valve has a pressure sensitive member built therein for sensing the pressure (suction pressure) in the suction chamber. In the variable capacity compressor using such a capacity control valve, mechanical feedback control (suction pressure control) is carried out by means of the pressure sensitive member such that the suction pressure approaches a set suction pressure.
More specifically, the pressure sensitive member is constituted, for example, by a bellows or a diaphragm. Where a bellows is used as the pressure sensitive member, a compression spring is disposed within the bellows whose internal pressure is kept at a vacuum or atmospheric pressure, and one end of the bellows is acted upon by the suction pressure from outside. Thus, as the suction pressure decreases, the bellows serving as the pressure sensitive member extends.
The valve element of the capacity control valve is so arranged as to be applied with an electromagnetic force exerted by a solenoid as well as with a pressing force that the bellows as the pressure sensitive member produces when extending. While the amount of current supplied to the solenoid is fixed, the opening of the capacity control valve varies such that the suction pressure is kept at a set suction pressure determined by the amount of current supplied.
Meanwhile, Document 1 (Japanese Laid-open Patent Publication No. 10-38717) discloses a method of detecting the torque of a variable capacity compressor during the suction pressure control. In Document 1, the torque of the variable capacity compressor is calculated from the amount of current supplied to the solenoid of the capacity control valve and the outside air temperature. The calculated torque of the variable capacity compressor is used for controlling the rotational speed of the automotive engine.
In the suction pressure control, the amount of current (capacity control signal) to be supplied to the solenoid of the capacity control valve is determined on the basis of the set suction pressure. Where an actual suction pressure is higher than the set suction pressure by a certain amount, however, the discharge capacity of the variable capacity compressor can possibly rise to a maximum value if the determined amount of current is supplied to the solenoid.
The maximum value is determined by a mechanical constraint, namely, the tilt limit of the swash plate of the conversion mechanism. When the discharge capacity is at the maximum value, the discharge capacity naturally cannot be increased further even if the amount of current supplied to the solenoid is adjusted. In conventional systems, however, the solenoid is supplied with an amount of current exceeding the current supply amount (maximum-capacity current supply amount) with which the discharge capacity is maximized. This situation occurs because the amount of current supplied to the solenoid is determined taking account of the set suction pressure alone, and not the mechanical constraint on the maximum value of the discharge capacity.
While the solenoid is supplied with an amount of current exceeding the maximum-capacity current supply amount, there is no correlation between the amount of current supplied to the solenoid and the torque of the variable capacity compressor. If the correlation is lost, the torque of the variable capacity compressor cannot be accurately calculated by the torque detection method disclosed in Document 1.
As a result, instability of the engine speed control or engine stall may possibly be caused during the idling of the vehicle, for example.
In addition, if the torque of the variable capacity compressor cannot be accurately calculated, it is difficult to attain the original purpose of applying the variable capacity compressor to an air conditioning system, namely, to reduce the motive power consumed by the air conditioning system or the vehicle and thereby save energy as countermeasures against global warming.